When a virus infected to a computer,
It makes many changes to the computer, to the OS and also to applications.
Those affects are really bad.
You may have removed the virus completely by somehow, but you haven't fix the changes done by the virus.
And you can't fix them, because there is no method to find those changes.
First those changes reduce the performance.
Then they cause to malfunctioning in OS and as well as applications.
These malfunctioning again reduce the performance.
Now don't expect good performance from that computer. Its performance must be really bad.
Now there is only one solution.
Format the system drive and re-install OS with all the applications,
Then install a perfect virus guard.